Hydraulic fluids serve as the power transmission medium in a hydraulic system and are designed to transmit force and motion in industrial hydraulic systems. A very high percentage of industrial non-mobile hydraulic systems and mobile vehicles such as automobiles, tractors and earthmovers are now equipped with some type of semi-automatic or fully automatic transmission. These hydraulic systems and transmissions must be provided with a supply of a “functional” fluid that serves at least a function of a power transmitting medium, a hydraulic control fluid, a heat transfer medium, and a satisfactory lubricant. The problem of shrinkage of seals, particularly elastomeric seals, upon contact with functional fluids is important since such shrinkage causes leakage of the functional fluid which can lead to defective operation of the hydraulic equipment and vehicles.
To eliminate this problem, it is conventional to include in the functional fluid an additive whose presence therein causes the seal to swell. A number of such additives are known in the art. U.S. Pat. No. 4,029,588 discloses a substituted sulfolane in which one of the substituents is a 3-alkoxy or 3-alkylthio group, or the like, for swelling seals in machinery in an amount of 0.05-20.0 wt. %, and preferably in an amount of 0.1-5.0 wt. %. U.S. Pat. No. 4,116,877 discloses a functional fluid comprising a mineral lubricating oil base and 5-20 wt. % of seal swell agents comprising an oil-soluble tris(C8-C24 hydrocarbyl) phosphite ester and an oil-soluble C8-C24 hydrocarbyl substituted phenol, whereby the elastomer compatibility of the fluid is enhanced. Seal swell additives are quite often used in undesirably large quantities in the functional fluid and often have several disadvantages. Some are toxic. Some would lower or raise the viscosity of fluid and or/impair its oxidation stability. Some when added in a large amount will cause the seal to swell against the shaft or hydraulic cylinder rod, leading to increased seal wear.
In a number of patent publications and applications, i.e., US 2006/0289337, US2006/0201851, US2006/0016721, US2006/0016724, US2006/0076267, US2006/020185, US2006/013210, US2005/0241990, US2005/0077208, US2005/0139513, US2005/0139514, US2005/0133409, US2005/0133407, US2005/0261147, US2005/0261146, US2005/0261145, US2004/0159582, U.S. Pat. No. 7,018,525, U.S. Pat. No. 7,083,713, U.S. application Ser. Nos. 11/400,570, 11/535,165 and 11/613,936, which are incorporated herein by reference, a Fischer Tropsch base oil is produced from a process in which the feed is a waxy feed recovered from a Fischer-Tropsch synthesis. The process comprises a complete or partial hydroisomerization dewaxing step, using a dual-functional catalyst or a catalyst that can isomerize paraffins selectively. Hydroisomerization dewaxing is achieved by contacting the waxy feed with a hydroisomerization catalyst in an isomerization zone under hydroisomerizing conditions. The Fischer-Tropsch synthesis products can also be obtained by well-known processes such as, for example, the commercial SASOL® Slurry Phase Fischer-Tropsch technology, the commercial SHELL® Middle Distillate Synthesis (SMDS) Process, or by the non-commercial EXXON® Advanced Gas Conversion (AGC-21) process.
There is a still a need for functional fluids, particularly functional fluids employing Fischer Tropsch base oils, that are capable of operating over a wide temperature range, possess a high degree of oxidation resistance, be free of corrosive action, have foam control, have satisfactory low temperature fluidity, retain a useful viscosity at high temperatures, requiring minimal or no seal swell additive while still retaining excellent transmission seal compatibility.